This is a late response but I have 64G on my iMac and although I don't usually use all 64G on every cue I do end up using in the 50+ ballpark quite often so I think your choice to go with 64G is a wise one!

I think you should get a motherboard, which supports 128GB RAM, and first buy 64GB. I did that, and I'm very happy, because I had to buy more ram after 64GB. My templates takes max 80GB ram at the moment. And also ram is quite cheap nowadays.

If by "live playing" you mean loading up just one sound at a time and playing solely that sound, then I don't see why 8GB wouldn't be enough. But most people load at least a set's worth of sounds at once; so in that case, you may very well need more RAM, if most of the VIs you're using are sample based. On the other hand, if most of your sounds are modeled, 8GB should be enough to get you through the entire performance. There are certainly people gigging with 8GB laptops.

IDK. I don't think so, really. The time we put into becoming half decent composers is by far the biggest cost to trying on music as a profession, a cost that people often ignore.

Anyone -- including you, @Wolfie2112 you are obviously a smart guy -- who is intelligent and determined enough to write complex music to picture is also smart enough to be an accountant at a high level, a banker or other finance person, or maybe a lawyer or computer science jockey, and have a far more predictable income that ramps up more quickly as well. So the opportunity cost of spending 10 years getting established in this business could easily be reckoned at a million dollars or more, given that plenty of relatively low-level finance folk make way over $100k.

That's why the cost of this or that library or plugin, when viewed this way, is negligible, as is the cost of two or three computers. In the long run if it accelerates your path a little bit or helps you avoid a bottleneck on your first big project, the time saving / hassle saving / interruption in creative flow saving dwarfs the outlay on gear.